In follow-up of recent work by MacMahon, Joly, and their colleagues (1,2), showing that women with delayed first childbirth have an increased risk of breast and ovary cancers, we initiated an epidemiologic study of case-control design to explore relationships of reproductive capacity and altered risk of these two cancers. Special attention was directed to the question of whether increased cancer risk results from late first childbirth due to lower fertility or to voluntary (planned) delay. Another aim of the study would assess possible relationships of female steroid hormones, such as contraceptives and postmenopausal replacement drugs, to breast and ovary cancers. Interviews with breast and ovary cancer patients, and with age - and race-matched control subjects (allocation ratio of 1:2) from the same hospitals are being conducted in the five counties of the Bay Area. A mechanism has been developed for in-hospital case-notification of cancer patients that is backstopped with reporting from our population-based (3.3 million people) registry. Procedures for identifying appropriate control subjects from hospital rosters have been developed. Interviews are conducted in hospitals or at home to elicit: a detailed early menstrual, marital, and reproductive history of the patients; a brief pregnancy history for patient's mother, sisters and daughters; detailed probing questions regarding birth control practices prior to birth of the first child; and whether medical consultation was sought if a desired first pregnancy was delayed.